Generally, the overhead transmission cable has been used for transmitting the electric power generated in power plants to the primary substations in the remote central and adjacent receiving areas.
The conventional overhead transmission cable is composed of a central strength member 11, and a conductor unit 13 surrounding the central strength member 11, as shown in FIG. 1. Conventionally, the overhead transmission cable generally includes a central strength member mainly composed of a steel wire and a steel cord, and a conductor unit composed of an aluminum or an aluminum alloy, and it is usually referred to as an aluminum conductor steel reinforced cable (ACSR).
Such a conductor unit 13 of the overhead transmission cable functions to transmit electric current, wherein a circular or pressed aluminum conductor may be used in an outside of the strength member, and such a conductor unit may be formed in multiple layers.
Meanwhile, the strength member 11 arranged in a central region of the overhead transmission cable functions to support the transmission cable, as well as to maintain its cable strength. The structure of such a central strength member may be in the form of a solid wire, or a stranded wire composed of several solid wires.
Generally, the overhead transmission cable is installed outdoors by hanging on the supports such as a plurality of steel towers or electric poles installed at predetermined intervals, but the strength member of the overhead transmission cable should be excellent in physical properties such as tensile strength, and have high tension and low-sag characteristics due to such environmental properties.
However, the overhead transmission cable is exposed to the external environment and used under such rather severe conditions, for example temperature of the cable itself is increased to 90° C. or more when the electric current is transmitted through the cable. In particular, the heat generated by transmission of the high-voltage current may inflate the central strength member supporting the overhead transmission cable, which causes the cable to be drooped.
Especially, the strength member composed of the steel cord and the steel wire, which has been used in the prior art, is heavy-weight, so the drooping phenomenon of the cable is more seriously increased and also steel towers and electric poles are heavily subject to the extreme press, which causes a safety problem.
Such problems have been made worse as the transmission capacity recently increases. Therefore, the measures should be taken to install taller steel towers or electric poles and reduce installation intervals of the steel towers or the electric poles, considering the drooping phenomenon of the cable at a high temperature.